Membrane distillation (MD) is a thermally driven separation process that has been investigated widely for many applications including, but are not limited to, water desalination, food processing and removal of volatile organic compounds from water [1]. Many recent review articles have summarized research that has done in the field of MD [1-6].
The principle of MD is based on applying a thermal gradient between both sides of a porous hydrophobic membrane that acts as a physical support separating a hot feed solution from a cooling chamber containing either a liquid or a gas depending on the used MD configuration. For instance, in direct contact membrane distillation (DCMD), a cold liquid solution is allowed to flow through the permeate side of the membrane in order to condense the vapour that has migrated through the membrane pores from the hot feed solution. Other MD configurations can be used to recover and condense the migrated vapour molecules: vacuum membrane distillation (VMD), sweeping gas membrane distillation (SGMD) and air gap membrane distillation (AGMD) [1-6].
The main advantages of MD, compared to other desalination processes, are the high selectivity for non-volatile compounds (100% retention of ions, macromolecules, colloids, etc.). In addition, it provides the possibility of working at low temperatures, which allows coupling to low-grade, waste or alternative energy sources [7]. In spite of these obvious advantages, a MD process is not commercialized yet for large scale desalination plants. The reasons behind that are the relatively lower MD flux compared to the production of the well established commercialized desalination processes such as reverse osmosis and the membrane wetting which diminishes the durability of MD membranes. In other words, there is a lack of adequately designed MD membranes, which should have low conductive heat flux (i.e., low heat loss by conduction through the membrane matrix) and high mass flux in order to increase the membrane flux as well as small pore size to decrease the danger of membrane wetting.
The membranes that have been used in MD are, generally, porous membranes made of hydrophobic material such as polypropylene (PP), poly(vinylidene fluoride) (PVDF) and polytetrafluoroethylene (PTFE), available in capillary or flat-sheet forms, although these membranes were marketed for microfiltration and ultrafiltration processes [6].
Recently, in MD research, more attention has focused on preparing membranes specifically for the MD applications [7-19]. The objective of the research was either to increase membrane durability or improve permeation flux. For example, Peng et al. [7] prepared composite flat sheet MD membrane by casting a hydrophilic polymer on PVDF hydrophobic substrate. The membrane was tested by DCMD configuration and the results were compared to the PVDF uncoated substrate. The coated membrane durability was improved compared to the uncoated membrane; although the flux decreased by about 9% (a flux of 23.7 kg/m2·h was achieved by the new membrane at feed and coolant temperatures of 70° C. and 12° C., respectively). Feng et al. [8] prepared asymmetric flat-sheet membranes from poly(vinylidene fluoride-co-tetrafluoroethylene) by the phase inversion method. Those membranes were tested by DCMD configuration and the results were compared to PVDF flat-sheet membranes prepared by the same procedure. Their new membranes exhibited higher flux than those of the PVDF membranes. They also prepared membranes from poly(vinylidene fluoride-co-hexafluoro propylene) [9] and found that the DCMD performance of these membranes was better than that of the PVDF membrane. Li and Sirkar [10] and Song et al. [11] designed novel hollow fiber membrane and device for desalination by VMD and DCMD. The membranes were commercial polypropylene (PP) membranes coated with plasma polymerized silicone fluoropolymer. Permeate fluxes as high as 71 kg/m2·h at 85° C. feed temperature using VMD configuration were achieved. The same type of membranes was used for larger scale DCMD desalination device [12]. Bonyadi and Chung [13] used the co-extrusion method to prepare dual layer hydrophilic/hydrophobic hollow fiber membranes for MD. PVDF was used as a host polymer in the spinning dope, where hydrophobic and hydrophilic surfactants were added. A flux as high as 55 kg/m2·h at inlet feed and permeate temperatures of 90° C. and 16.5° C., respectively, was achieved using DCMD configuration.
In series of publications [15-19], the requirements of higher flux MD membranes were clearly identified. As a result, the concept of hydrophobic/hydrophilic composite membranes for MD was first presented by Khayet et al. [15, 16]. It was shown that this type of membrane satisfies all the requirements of higher flux MD membranes [16, 17]. The hydrophobic/hydrophilic membrane was prepared by phase inversion method in a single casting step. A hydrophilic base polymer was blended with a hydrophobic surface modifying macromolecules (SMMs). During the casting step, the SMMs migrated to the air/polymer interface since they have lower surface energy [20]. Consequently, the membrane top-layer became hydrophobic while the bottom layer became hydrophilic.
There remains a need for high flux membranes for use in MD.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.